This invention relates to alkaline secondary electrochemical cells which utilize zinc as the active electrode material and, more particularly, it relates to the use of mercury-free zinc electrodes in such cells.
Zinc is used as the negative electrode material in alkaline electrochemical cells for a number of reasons including its high half-cell voltage, its low polarization, and its high limiting current density on discharge. However, a number of adverse effects result from the use of zinc in alkaline cells. Because it is thermodynamically unstable in alkaline media, zinc self-discharges significantly while standing. Such discharge is accompanied by hydrogen gas evolution. Together, these reactions produce electrode corrosion and shortened shelf life.
In order to overcome the aforementioned disadvantages while still taking advantage of the benefits derived from the use of zinc, battery manufacturers normally include a small amount of a material capable of raising the hydrogen overvoltage of zinc, e.g., 1-4% of mercury in the form of HgO, in zinc electrodes to reduce self-discharging of the latter and to suppress evolution of hydrogen gas.
Although the use of mercury in zinc electrodes is advantageous, its presence in such cells and the processes associated with the introduction of mercury into such cells present both environmental and health hazards. Therefore, it would be desirable to be able to produce mercury-free zinc electrodes having electrochemical characteristics similar to or better than mercury-containing zinc electrodes.
Inorganic additives other than mercury have been included in zinc electrodes for some time in order to reduce the corrosion of such electrodes by reducing their self-discharge in alkaline solution. Suggested additives have included: lead and lead compounds such as lead oxide, lead hydroxide, lead sulfide, and lead acetate; indium hydroxide; stannous chloride; cadmium oxide and cadmium acetate; and thallium oxide. The aforementioned additives may be used in absence of, or in combination with, mercury in zinc electrodes. Exemplary of the use of such additives in zinc electrodes are U.S. Pat. Nos. 3,639,176, 3,642,539, 3,847,669, 3,816,178 and 3,785,868. Although these additives have been found to be partially effective in reducing self-discharge of zinc electrodes in alkaline media, the same additives may adversely affect the potential-current curve on discharge, particularly at high battery drain rates.
In order to utilize the various additives in zinc electrodes, binders such as carboxymethyl cellulose, polyvinyl alcohol and methyl cellulose have typically been used. They facilitate introduction of the additive into the zinc electrode and retain the additive in the electrode. However, use of most of those binders produces adverse effects as a result of precipitation reactions taking place between the binder and additive. In particular, otherwise useful lead acetate and cadmium acetate additives precipitate with carboxymethyl cellulose and sodium carboxymethyl celluose to prevent homogeneous dispersion of the additive in the electrode. Without such homogeneous dispersion, localized hydrogen gas evolution can still occur throughout the electrode. Furthermore, others of the aforementioned binders cannot be used successfully with lead and cadmium acetate because they also cause a "salting out" effect and massive precipitation reactions.